(i) Field of the Invention
The invention relates to a method and a device for delivering a pure gas charged with a predetermined quantity of at least one gaseous impurity to an apparatus, especially a trace-impurity analyzer such as an atmospheric-pressure ionization mass spectrometer.
(ii) Description of Related Art
In the field of the analysis of very-high purity gases, analyzers designed to detect impurities at very low concentrations (for example, from 10.sup.-2 to 10.sup.-5 ppm, or even from 10.sup.-3 to 10.sup.-6 ppm) are often used. Such an apparatus requires calibration at regular time intervals in order to ensure reliable and accurate operation.
Such a calibration is carried out by sequentially delivering to the analyzer firstly a pure or "zero" gas, that is to say a gas containing less than 10.sup.-5 ppm of impurities, and then a so-called "calibration" gas containing impurities in precisely known concentrations.
Such methods and such devices designed for the feed of an analyzer having a very high sensitivity are known from document FR-A-2714968 in the name of the Applicant and from document U.S. Pat. No. 5,239,856, also in the name of the Applicant.
The aforementioned FR document describes a device and a method in which a pure gas is charged with trace impurities using a battery of permeation cartridges connected in parallel to a unit for diluting the impurities in the pure gas. Each cartridge contains one gaseous impurity, such as H.sub.2 O, CO.sub.2, CO, O.sub.2, CH.sub.4, H.sub.2, etc., for example, which diffuses in very small quantity through a diaphragm for charging the pure gas flowing into the dilution unit.
This device has the drawback that each permeation cartridge creates, through its connection line to the dilution unit, a branch in which the flow stagnates, causing undesirable interactions with the walls of the line, such as, for example, absorption and desorption phenomena.
In addition, these connection branches are difficult to purge, compromising the use of such a battery of permeation cartridges with various kinds of pure gases.
Moreover, the flow rate of the impurity introduced through a permeation cartridge varies over time. Consequently, the accuracy with which the composition of the calibration gas is known decreases with time.
Another drawback of the device is connected with the operation of the permeation cartridges. A permeation cartridge is produced by a reservoir which includes an outlet closed off by a membrane, for example a silicone membrane, through which the impurity contained in the reservoir diffuses.
However, this membrane is not only permeable in the cartridge/dilution unit direction but also in the reverse, dilution unit/cartridge direction.
Thus, the pure gas in the connection branch can diffuse through the membrane into the cartridge, especially when the molecule is small, such as the molecule H.sub.2. In the course of time, an unknown gas composition is created in the permeation cartridge, consequently compromising the precise generation of a calibration gas, especially in the case of the sequential use of the device with various kinds of pure gases.
Also currently, a procedure is carried out in the following manner: by using pure-gas/impurity I gas mixtures made up beforehand, the impurity I having a content which may be termed intermediate content, for example of the order of magnitude of 1 ppm.
Thus, for example, premixed bottles containing a few tens of ppm of CO or of hydrogen in nitrogen are used.
The required dilution steps are then performed, using the corresponding pure gas (Ar, N.sub.2, He, etc.), of the gas in each premixture bottle, in order to end up with the final mixture containing the impurity I in the pure gas in question with a content, for example, of the order of magnitude of one ppb.
It may therefore be imagined that such a procedure is complex, expensive and causes multiplication of the sources of errors and contamination of the mixtures in the various manufacturing steps.
As regards document U.S. Pat. No. 5,239,856, this describes a method and a device for manufacturing a pure gas charged with trace impurities, according to which a certain quantity of gas is sampled from two gas sources, each gas of which constitutes an impurity in a pure gas.
The sampled gases are mixed and the mixture diluted in a dilution unit so as to obtain a very low concentration of the impurities in the pure gas, said concentration lying within a range extending from 10.sup.-5 ppm to 10.sup.-2 ppm, so as to deliver this calibration gas to a trace impurity analyzer which must be calibrated.
The accuracy with which the concentrations of the trace impurities in the pure gas are known depends not only on the quality of the dilution unit but also on the quality of manufacture of the gas mixture. However, in the case of the aforementioned US-A document, this accuracy depends on the uncertainty in the setting of the various mass flowmeters placed in the impurity sampling lines. By increasing the number of impurity sources and, consequently, the number of mass flowmeters for manufacturing the mixture, the accuracy with which the composition of the gas mixture is known decreases since the uncertainties in the flow rate of each mass flowmeter add together to form the uncertainty in the composition of the gas mixture obtained.